1. Field of the Invention
The present invention relates to a hydraulic transmission with a hydraulic pump and a hydraulic motor fluidly connected thereto for driving and steering wheels.
2. Related Art
Up to the present, various hydraulic transmissions, which are capable of steering driving wheels of a vehicle while driving the same by fluidly connected hydraulic pump and hydraulic motor have been proposed. For example, PCT Application Publication No. WO00/01569 discloses a hydraulic transmission of this type, which includes a first HST (Hydrostatic Transmission) having a first variable displacement hydraulic pump operatively connected to a driving source and a first hydraulic motor fluidly connected to the first variable displacement hydraulic pump, and a second HST having a second variable displacement hydraulic pump operatively connected to the driving source and a second hydraulic motor fluidly connected to the second variable displacement hydraulic pump, in which an output shaft of the first hydraulic motor is connected to a pair of driving axles via a first differential unit, and an output shaft of the second hydraulic motor is operatively connected to the pair of driving axles via a second differential unit.
In the thus arranged hydraulic transmission, the first HST acts to transmit power with its speed variable to both a pair of driving wheels, while the second HST acts to steer both the pair of driving wheels. This arrangement can achieve an improved operability and the like, as compared with the other type of the hydraulic transmission, in which both a pair of the HSTs are used in order to drive respectively the left and right driving wheels with variable speed.
Thus, the hydraulic transmission as disclosed in the above PCT International Publication has an advantageous effect than a conventional hydraulic transmission, but is also disadvantageous in the following respects.
That is, the hydraulic transmission necessitates the fully independent paths, which respectively constitute the first HST for driving the wheels with variable speed and the second HST for steering the wheels, and therefore requires to provide such as gear transmission mechanisms including differential units, their associated parts and housings for accommodating these members between the first HST and the pair of the output shafts, as well as between the second HST and the pair of the output shafts. As a result, a layout possibility of the vehicle is limited and hence design flexibility is deteriorated.
In consideration of the above prior art, it is an object of the present invention to provide a hydraulic transmission equipped with the fluidly connected hydraulic pump and hydraulic motor, which is simple in construction and capable of transmitting power to the driving wheels with variable speed and steering the same, as well as achieving improved design flexibility in the layout of the vehicle.